Three Part Question

In [children presenting with acute hip pain], is there [a single clinical or laboratory test] that will [distinguish between septic arthritis and
transient synovitis]?

Clinical Scenario

A 3 year old boy presents to paediatric A&E with refusal to weightbear for the last day and a temperature of 37.9 ° centigrade. The pain is located in his hip. What clinical or laboratory tests could help discriminate between septic arthritis and transient synovitis?

Three-variable model of history of fever, serumWCC and prior health care visit gave predicted probability of 71%

Retrospective; small SA sample, lacks power
No CRP; high percentage of TrS suggests low threshold for hip aspiration

Jung et al,2003,Korea

127 children with acute hip pain (27 with SA, 97 with TrS)

Level III (diagnostic study)

Predictors:

1. Temp > 37°C

2. sWCC>11

3. differential

4. ESR >20 mm/hr

5. CRP >10 mg/L

Predictors 1,2,3,4,5 all p<0.001, but lack of obvious cut-off point limits usefulness

Predicted probability of SA when 1,2,4,5 all present is 98.8%

Small SA sample
Retrospective
Complicated algorithm, not practical for clinical use
High number of TrS cases suggests low threshold for hip aspiration

Comment(s)

Distinguishing between septic arthritis and transient synovitis of the hip joint in the limping child can be a difficult clinical undertaking but is vital. The two conditions can present with similar symptoms and clinical features but the treatment and potential for negative sequelae are significantly different. Whereas transient synovitis runs a benign self-limiting course that can be managed with observation and NSAIDS, septic arthritis needs urgent diagnosis, operative irrigation and antibiotics. Poor outcomes are associated with diagnostic delays, and negative
sequelae include osteonecrosis of the femoral head, growth arrest and sepsis (Fabry, Lunseth).

There is much debate amongst clinicians over how best to accurately and quickly distinguish septic arthritis from transient synovitis with no one pathognomonic symptom, clinical feature, blood test or investigation confirming the diagnosis. If there is sufficient clinical suspicion of septic arthritis, the hip joint must be aspirated under image guidance (ultrasound or fluoroscopy) and the fluid sent for laboratory examination. Joint aspiration is considered the gold standard test
but is an invasive and unpleasant procedure, particularly for a child, usually requiring general anaesthesia. The question is, ‘‘what amounts to
sufficient clinical suspicion?’’. How much weight should be attached to each clinical finding or investigation result? There is therefore a need for a clinical prediction rule (CPR) to aid the clinician to safely distinguish between the two diagnoses and avoid both delayed treatment and
over-investigation.

Kocher’s (1999) clinical prediction model to differentiate between septic arthritis and transient synovitis forms the basis of subsequent validation studies by Luhmann (2004), Caird (2006) and Kocher (2004).Kocher (2004) prospectively evaluated 154 patients and found that the same four variables were still most likely to predict the outcome of septic arthritis, with a diminished, but nevertheless good, diagnostic performance in the new patient population (93% probability with all four predictors present).

Luhmann et al, in another tertiary children’s hospital, applied the same proposed CPR retrospectively on all children who had undergone aspiration of the hip joint as part of their work-up for acute hip pain. Despite similar patient demographics, they found a lower predicted probability of septic arthritis (59%) when all four variables were present. These findings emphasise the value of validating all clinical prediction rules prior to application in clinical practice. Caird et al published a validation study of Kocher’s CPR, including 48 children presenting with signs and symptoms suspicious enough to warrant ultrasound-guided
hip aspiration. The findings supported Kocher’s model to a certain extent: the likelihood of a patient having septic arthritis increased with the number of positive factors; however, the predicted
probability of septic arthritis with none of the predictors present was still 16.9%. The retrospective derivation study by Jung et al included 97 patients with transient synovitis and 27 with septic arthritis. There are two main issues with their methodology. Firstly, not all patients included in the study underwent the golden standard investigation. Secondly, only patients with a positive synovial culture were definitely diagnosed with septic arthritis, oddly leaving joints with a positive microscopy (ie, synovial aspirate white cell count .50 000 ml21)but negative culture possibly classified as transient synovitis. Finally, Jung’s rule has not been externally validated.

In summary, Kocher’s (1999) proposed four predictor clinical decision rule remains the most useful clinical tool to date despite inadequate external validation. CRP measurement may have an additional benefit as suggested by Caird et al. The study by Jung et al has sufficient methodological flaws as to limit its usefulness.